This invention relates to nuclear fuel assemblies and, more particularly, to an improved nuclear fuel assembly including spring means which acts to hold a resilient in-core detector tube concentrically within an instrumentation tube to retard wear imposed on the detector tube due to coolant flow induced vibration.
In most pressurized water reactors, the reactor core is composed of a large number of elongated fuel assemblies which receive support and alignment from upper and lower transversely extending core plates. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles to form an integral fuel assembly. The guide thimbles extend slightly above and below the ends of the fuel rods. Fuel assemblies also employ instrumentation tubes to accommodate a resilient in-core detector tube in which an in-core detector is reciprocalloy movable. The instrumentation tube is located centrally of each fuel assembly and has its lower end secured to the bottom nozzle in alignment with a bore formed in the center thereof.
During reactor operation, the in-core detector is controllably inserted into a selected one of the detector tubes to monitor neutron flux density at selected axial locations. A change-over system functions to selectively insert such in-core detectors into a number of the detector tubes according to a predetermined monitoring schedule. When the detector tube is maintained in the instrumentation tube, it experiences movement, or vibration, relative to the guide tube and the bore in the bottom nozzle. Coolant flow occurring between the bottom nozzle and the lower core plate is a source of vibration and such flow induced vibration is a cause for wear along the detector tube which abuts the surrounding tube or inner circumference of the bore. Such wear can influence the useful life of the in-core detector tube. Once the outer wall of the detector tube has been worn to a certain depth at one location, the tube must be replaced. Therefore, if such wear can be retarded, the useful life of the detector tube can be increased.
Accordingly, it is the principal object of the invention to provide an improved nuclear fuel assembly including spring means which can retard wear imposed on the in-core detector tube by holding the detector tube concentrically within the instrumentation tube.
It is another object of the invention to provide an improved nuclear fuel assembly including an extension sleeve which effectively increases the axial length of an in-core detector tube that will likely abut the surrounding sleeve to thereby retard wear along the tube surface.
It is still another object of the invention to provide an improved nuclear fuel assembly wherein the inner circumferential surface of the bore in the bottom nozzle is formed of a hardened material to decrease the rate of wear in the outer surface of an in-core detector tube so as to prolong the useful life of the detector tube.